1. Field of the Invention
The present invention relates to apparatus and methods for electronically reading and transmitting documents and microfilmed images. More particularly, this invention pertains to a new electronic camera for processing information as sensed by a charge coupled device (CCD).
2. Description of the Prior Art
The advantages of electronic processing in the storage and transmission of document-type images are well-recognized. Such storage may take numerous forms including magnetic (tape or disc) and optical (laser disc) media while transmission may take place by cable, radio and telephone links and the like. Such processes, unlike photographic processes, are recognized to be economical, instantaneous and impervious to aging or deterioration.
In the electronic processing of images, a promising sensor technology, as a replacement for the Vidicon and like camera tubes, is the charge coupled device (CCD). This solid state sensor does not introduce geometric distortions into the image, it is compact, and it requires no sophisticated electronic circuits for actuation.
The output of the CCD sensor is an analog voltage. This analog value often must be converted to digital format. Such analog-to-digital conversion is especially pertinent when a binary photocopy-like output is desired. In such a case, no "gray" shades are employed. Every pixel of the image is either "black" or "white".
The conversion of a "real-life" image to binary format inherently requires the application of "intelligence". That is, for each pixel that is neither black nor white (i.e. a gray pixel), a decision must be made assigning one or the other value. In the case of typewritten documents, such decisions are generally not difficult. It is often adequate in such cases to assign white to any pixel having a value in the upper half of a light scale and black to any pixel having a vallue in the lower half of such a scale. The dividing line is known as a "fixed decision level" and the above-described technique is common to a rather "primitive" camera.
The entire process is complicated when colors are employed or when reading film that is neither as transparent or opaque as expected. Defocussing can alter the opacity of a thin black character element. The presence of fingerprints and scratches can modify light and dark values. Attempts have been made to compensate for such anomalies by local modification of the decision level for the scanned area. Among the techniques for effecting the decision level is that of "dynamic thresholding" as shown, for example, in United States patent Ser. No. 4,399,470. In that patent there is disclosed an analog circuit for storing the light values of the pixels immediately preceding and following the "decision pixel". The threshold value employed is arrived at through the simple averaging of the two proximate values. Such a method is inherently limited and sometimes incapable of effective and accurate resolution of "tricky" images.